Substitution Group Effects on Gas Sorption and Diffusion into Polyimides

Abstract

The sorption and diffusion of carbon dioxide below 1 atm were measured for seven polyimide films by a volumetric sorption method and a permeation method. The diffusivities of carbon dioxide were correlated with packing density (reciprocal of specific free volume) for ail except two polyimides. When the packing density of the polymer was increased, the diffusivities decreased. But, as the packing density was increased further, the diffusivities increased. The substituent sulphone = SO ₂ promotes hydrogen bonding and causes the polymer main chain to pack densely. Consequently the gas diffusion of this polymer is rather low. The packing density of these two polymers may not be successfully calculated by Bondi's method. However, all die diffusion coefficients decrease linearly with an increase in the cohesive energy density (CED) which is calculated by the group contribution method of van Krevelen. These results suggest that the gas diffusions of polyimides correlate with CED more than with the packing density.